Proposals have been made in published French patent application number 2 629 956 for a limiter comprising two superconducting windings in parallel, each carrying equal current.
In U.S. Pat. No. 3,703,664, proposals are made to use a hybrid coil having a superconducting winding and a non-superconducting winding. The resistance of the non-superconducting winding is very low relative to its reactance. Current is shared equally between the two windings which have about the same number of turns. In the event of a fault, the switch-over or "transition" of the superconducting winding causes nearly all of the current to switch to the non-superconducting winding which behaves substantially like a pure inductance, thereby limiting the fault current.
The above-mentioned devices are not suitable for performing rapid reclosure cycles because of the relatively long period of time taken to return the superconducting winding to the superconducting state after a transition. The greater the current normally conveyed, the longer the time taken.
To solve this problem and to enable rapid reclosure cycles to be performed, proposals are made in French patent application number 90 03 398 to provide a limiter comprising a superconducting first winding and a non-superconducting second winding, the two windings being wound in opposite directions and being connected in parallel, with the number of turns in the superconducting winding being greater than the number of turns in the non-superconducting winding, and with the superconducting winding carrying a fraction of the total current, e.g. about 20%.
In such a limiter, a fault current greater than the critical current causes the superconducting winding to switch over so that its resistance increases suddenly. Nearly all of the current then passes through the non-superconducting winding, however the self-inductance of this winding limits the value of the current. Since the current normally conveyed is relatively low, and in spite of a higher restored voltage, the superconducting winding returns to the superconducting state fast enough to make rapid reclosure cycles possible.
In the limiter described above, the superconducting winding is housed in an insulating cryostat and it is wound inside the non-superconducting second winding. The insulating cryostat, e.g. filled with liquid helium, requires a certain thickness of insulation which does not facilitate the establishment of close electromagnetic coupling between the two windings. The inductive voltage loss across the terminals of the limiter in normal operation runs the risk of exceeding the acceptable threshold, i.e. about 5% of the phase-ground voltage.
An object of the present invention is to provide a device which avoids the above-mentioned drawback while conserving the property of making rapid reclosure cycles possible.